# 13: Streams and Floods (Exercises)

- Page ID
- 2052

### Q13.1 How Long Does Water Stay in the Atmosphere?

*The volume of the oceans is 1,338,000,000 km ^{3} and the flux rate is approximately the same (1,580 km^{3}/day). *

What is the average residence time of a water molecule in the ocean?

*1,338,000,000/1,580 = 846,835 days average residence time for water in the ocean (or 2320 years)*

### Q13.2 The Effect of a Dam on Base Level

How does the formation of a reservoir affect the stream where it enters the reservoir, and what happens to the sediment it was carrying?

*The velocity of the streams slows to zero and most of the sediment is deposited quickly.*

The water leaving the dam has no sediment in it. How does this affect the stream below the dam?

*With nothing to deposit, the water below the dam can only erode, so there will be enhanced erosion below the dam.*

### Q13.3 Understanding the Hjulström-Sundborg Diagram

- A fine sand grain (0.1 mm) is resting on the bottom of a stream bed.

(a) What stream velocity will it take to get that sand grain into suspension? *~20 cm/s*

(b) Once the particle is in suspension, the velocity starts to drop. At what velocity will it finally come back to rest on the stream bed? *~1 cm/s*

- A stream is flowing at 10 cm/s (which means it takes 10 s to go 1 m, and that’s pretty slow).

(a) What size of particles can be eroded at 10 cm/s? *No particles, of any size, will be eroded at 10 cm/s, although particles smaller than 1 mm that are already in suspension will stay in suspension.*

(b) What is the largest particle that, once already in suspension, will remain in suspension at c0 cm/s?* A 1 mm diameter particle should remain in suspension at 10 cm/s.*

### Q13.4 Determining Stream Gradients

The length of the creek between 1,600 m and 1,300 m elevation is 2.4 km, so the gradient is 300/2.4 = 125 m/km.

- Use the scale bar to estimate the distance between 1,300 m and 600 m and then calculate that gradient.
*5.2 km, with a gradient of 700/5.2 = 134 m km* - Estimate the gradient between 600 and 400 m.
*3.6 km, with a gradient of 200/3.6 = 56m /km* - Estimate the gradient between 400 m on Priest Creek and the point where Mission Creek enters Okanagan Lake.
*4 km, with a gradient of 60/4.0 = 15 m/km*

### Q13.5 Flood Probability on the Bow River

- Calculate the recurrence interval for the second largest flood (1932, 1,520 m
^{3}/s).*Ri = 96/2 = 48 years* - What is the probability that a flood of 1,520 m
^{3}/s will happen next year?*1/48 = 0.02 or 2%* - Examine the 100-year trend for floods on the Bow River. If you ignore the major floods (the labelled ones), what is the general trend of peak discharges over that time?
*In general the peak discharges are getting lower (from an average of around 400 m*^{3}/s in 1915 to an average of about 300 m^{3}/s in 2015)